A typical integrated circuit comprises a semiconductor substrate and a back-end structure formed on one surface of the semiconductor substrate. Transistors are formed in the semiconductor substrate and are connected together and to external connections by a network of interconnects that extends through insulating layers in the back-end structure. Further details about the back-end technologies in integrated circuits may be found, for example, in J. D. Plummer et al., Silicon VLSI Technology, Ch. 11 (Prentice Hall 2000) which is incorporated by reference herein.
Circuit elements such as resistors, capacitors, and inductors are frequently formed in the back-end structure. This concerns capacitors that are sometimes referred to as MIM (Metal-Insulating-Metal) capacitors. They are often used as decoupling capacitors.
In some cases it is desirable to connect MIM capacitors in series. One such series MIM capacitor comprises a first electrically conducting plate, second and third electrically conducting plates located side-by-side above the first plate, and an insulating layer between the first plate and the other two plates. Typically, the first plate has the same lateral extent as the second and third plates. External connections are made to each of the second and third plates.
Unfortunately, this arrangement causes one of the two capacitors to be forward biased while the other is reverse biased. It also has an asymmetrical leakage current which leads to concerns about its reliability.
In an alternative embodiment, two identical capacitors are formed each having lower and upper plates with an insulating layer between them; and the lower plate of the first capacitor is connected to the upper plate of the second capacitor using a laterally extending pathway located above the two capacitors. External connections are also made to the upper plate of the first capacitor and the lower plate of the second capacitor. To provide connection space on the surface of the lower plates, the lower plates must extend laterally beyond at least one edge of the upper plates. These lateral extensions and the spacing between the lower plates result in an inefficient use of available space.